Mold for casting single crystal articles

ABSTRACT

A mold construction for use in making single crystal castings in which adequate metal area is provided for heat conduction to the chill plate during the solidification which facilitates heat control to assure growth of a single crystal into and through the article cavity of the mold.

U United States Patent [191' 1 3,724,531 Erickson et al. [4 Apr. 3, 1973 ['54] Mom ron CASTING SINGLE [56] References cm CRYSTAL ARTICLES 1 UNITED STATES PATENTS [75] Inventors: John S. Erickson, Wallmgford;

Patrick Curran Manchester; 3,515,205 6/1970 Wickstrand ..164/353 William A. Owczarskl, Chesire, all of Con I Primary Examiner-J. Spencer Overholser r Assistant Examiner-John E. Roethel [73] Ass1gnee: United Aircraft Corporation, East Anomey charles warren Hartford, Conn.

22 Filed: Jan.13,1971 7] ABSTRACT 2 APPL 077 A mold construction for use in making single crystal 1 castings in which adequate metal area is provided for heat conduction to the chill plate during the solidifica- [52] US. Cl ..l64/361, l64/60 i which facilitates heat control to assure growth of [51] Int. Cl. ..B22c 9/02, 322d 25/06 a single crystal into and through the article cavity of [58] Field of Search ..1-64/60, 122, 125, 127,338, the mom MOLD FOR CASTING SINGLE CRYSTAL ARTICLES BACKGROUND OF THE INVENTION tion of the mold. The crystal selector is generally a nonlinear passageway from the starter zone to the article portion of the mold and for convenience this is frequently a helix-shaped passage terminating at the base of the article portion of the mold and having an open lower end communicating with the starter portion of the mold. This crystal selector is a relatively small passage and thereby limits the cross-sectional area of meta] through which heat can be conducted from the solidifying portion of the alloy within the article forming portion of the mold into the chill plate on which the mold is positioned.

One way to increase the heat flow to the chill plate is described in the co-pending application of GIAMEI ET AL., Ser. No. 42,423, filed June 1, 1970 having a comwhich is a cavity 12, the shape of the airfoil portionof the turbine blade and a cavity 14, the shape of the root portion of the blade. This article forming portion is .positioned vertically with the airfoil portion at the top and the root portion at the bottom. The root portion has a base wall 16 to which is attached a helical-shaped mold portion 18 having helical passage 20 therein. This passage extends from the root cavity of the blade downwardly into a starter cavity or growth zone 22, and serves to select one single crystal from the columnar grains growing within the starter cavity 22. Thus, as

the dendritic growth occurring during solidification reaches the article portion of the mold, the growth is in the form of a single crystal and the entire article becomes a single crystal of cast alloy.

At the top of the article mold is a riser portion 24, defining riser passages 26 therein. The starter cavity is defined by a starter mold portion 28, that extends from mon assignee with this application. In this type of mold the article portion. and the helix are enclosed by a surrounding shell spaced from the article mold to provide a cavity surrounding the article mold and into which additional metal is poured during the casting of the article in order to control the rate of solidification within the enclosed articleportion of the mold.

The co-p ending application of BARROW ET AL,

Ser. No. 63,143 filed Aug. 12, 1970, having a common assignee with this application describes an apparatus and method by which higher rates of solidification may be accomplished, and this involves a much greater rate of heat removal from the article mold. The principal feature of the present invention is a mold construction particularly adapted for use in high-rate solidification as described in this pending application.

According to the present invention, the mold has an article portion to the base of which is attached a crystal selector element such as a helix and surrounding this crystal selector is a starter zone defined by a downwardly extending mold element secured at its upper end substantially at the periphery of the base of and the starter cavity will be thinner than the,

remainder of the shell mold.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view through the shell mold the invention positioned on a chill plate.

FIG. 2 is a fragmentary sectional view'similar to FIG. 1 showing a modification.

DETAILED DESCRIPTION OF THE DRAWING The mold shown is adapted for making a turbine blade, and to this extent, it has an article portion 10, in

substantially the periphery of the base of the article portion down to and in contact with a chill plate 30. This portion of the mold is long enough to support the bottom end of the helix portion 18 at a distance above the chill plate, preferably from 1 to 1 )6 inches. Attached to the portion 28 of the mold'is a vertically extending tube 32, having a pouring cup 34 at the upper end. This tube and cup provide for filling the mold with molten alloy in making the cast article. l The mold is preferably formed by the shell mold technique, in which the mold is formed around a disposable pattern, preferably a wax pattern. In the usual formation of the mold, a wax pattern corresponding to the shape of the finishedcast article is provided and attached toit isa helical-shaped wax extension to form the passage 20. This is attached as shown-at the center of the base surface of the article pattern. A shell mold is then formed around this pattern by successively dipping, stuccoing and drying coats of mold material around the pattern. Preferably, the mold wall thickness built up at this time, is not the full thickness required to provide a mold strong enough to be used for casting.

.After the article mold with the helix thereon has been formed in this manner, wax extensions are attached to the upper end of the wax pattern for the article'for the purpose .of forming the riser passages 26, and a wax pattern is formed around the helix portion 18, in engagement with the base wall of the mold and projecting downwardly far enough to permit the formation of the mold portion 28 around this portion of the pattern. At the same time, a wax rod to form the pouring passages is connected to the wax pattern for the starting cavity. This assemblage is then treated as above by usual shell molding techniques to complete the mold in such a way as to provide the desired thicknessof mold wall surrounding the article portion of the first mold, and in intimate contact with it as shown and also to provide adequatethickness for the portion of thestarter cavity and pouring starter cavity. The chill plate is shown to indicate the position of the mold when the latter is ready for use in a casting process.

By fabrication of the shell mold as described above, the base wall of the article portion of the mold is relatively thin, since only the first or preliminary mold thickness occurs in this wall. Since there is alloy on both sides of this base wall when the mold is filled this thinner wall portion is not exposed to the same pressure load that the outer walls of the mold are exposed to. During solidification of the alloy within the mold, the portion of metal filling the starter cavity 22, serves to conduct heat from the base portion 16 of the mold downwardly to the chill plate, and thereby conducts heat at a much greater rate than would be possible if conduction took place only through the solidified alloy within the helical passage 20. It will be understood that a high rate of heat conduction downwardly from the article portion of the mold is essential in controlling the rate of solidification of the alloy to maintain the desired single crystal growth.

Although a particular arrangement of mold having a double wall construction surrounding the article portion is shown, it will be understood that a mold suitable forsome applications would be acceptable if the secondary mold, which is shown as enclosing the primary mold and also forms the starter portion 28 of the mold were merely an extension 32 downwardly from the periphery of the base portion 16 of the article mold 1.0 as shown in FIG. 2. In this event however, the first primary mold formed around the article portion, would of necessity be made thicker, in order to be strong enough for the casting operation.

The subject matter of this application is described 4 but not claimed in the above-identified co-pending application BARRQW ET AL., Ser. No. 63,143 having a common assignee with this application.

We claim:

1. A shell mold construction for use in the casting of single crystal articles including an article mold portion defining a cavity and having a base, a crystal selector mold element attached at the base of the article mold portion and having a helical passage therein, growth mold portion defining a growth cavity below the mold element and surrounding and spaced from the crystal selector mold element and in contact with the article mold portion substantially at the periphery of the base and extending downwardly from the entire periphery of the base portion to form the growth cavity substantially corresponding in area to the base of the article mold portion, said growth portion having a filling opening in the wall thereof below the article mold portion, and

said growth portion having an open lower end spaced from the bottom end of the selector mold element and adapted'to rest on a. chill plate.

2. A mold construction, as in claim 1, in which the article mold portion is a double wall construction with the walls in contact throughout, the growth mold portion is the thickness of the outer wall and integral with and extends downwardly from the outer wall, the base portion is inte he article po ion is the thickness of the inner wall and is thus thinner than the remainder of the article mold portion of the shell mold.

ral with the inner wall and the base of 

1. A shell mold construction for use in the casting of single crystal articles including an article mold portion defining a cavity and having a base, a crystal selector mold element attached at the base of the article mold portion and having a helical passage therein, a growth mold portion defining a growth cavity below the mold element and surrounding and spaced from the crystal selector mold element and in contact with the article mold portion substantially at the periphery of the base and extending downwardly from the entire periphery of the base portion to form the growth cavity substantially corresponding in area to the base of the article mold portion, said growth portion having a filling opening in the wall thereof below the article mold portion, and said growth portion having an open lower end spaced from the bottom end of the selector mold element and adapted to rest on a chill plate.
 2. A mold construction, as in claim 1, in which the article mold portion is a double wall construction with the walls in contact throughout, the growth mold portion is the thickness of the outer wall and integral with and extends downwardly from the outer wall, the base portion is integral with the inner wall and the base of the article portion is the thickness of the inner wall and is thus thinner than the remainder of the article mold portion of the shell mold. 